Buried Tank Installation
Tanks must be fully supported on the base by either an appropriate concrete pad that allows no overhang or a compacted stone-free sand bed.
When buried, tanks must be filled with water prior to backfilling. Tank tops are non-load bearing and should have a trap door or access cover over them supported by a foundation or by the ground. They cannot be supported by the top of the tank.
The backfill material shall be free of stones and foreign matter and shall be capable of passing a No. 10 screen. The depth of this backfill should be 6" to 8" thick around the tank and then normal backfill material may be used. Mechanical tamping is not recommended. For tanks with extensions, the above procedure should be followed up to or slightly above water level and from that point on up to ground level it is recommended that a 4" thick concrete sleeve be poured around the tank in multiple lifts. If this is not feasible then the normal backfill procedure may be used, up to 36" height above water level, bearing in mind that backfilling will produce inward pressures on the empty portion of the tank. Care must be exercised during this operation to prevent inward buckling.
A concrete vault must be used or a 4" thick concrete sleeve poured around the tank in multiple lifts in areas with high water tables or poor soil conditions. If the tank is to be used as a pump-out holding tank or similar type of operation or the tank will be left empty or partially full, then a vault is required.